The Art of Assembly Language

2.16 The Unicode Character Set

Although the ASCII character set is, unquestionably, the most popular character representation on computers, it is certainly not the only format around. For example, IBM uses the EBCDIC code on many of its mainframe and minicomputer lines. Because EBCDIC appears mainly on IBM's big iron and you'll rarely encounter it on personal computer systems, we will not consider that character set in this text. Another character representation that is becoming popular on small computer systems (and large ones, for that matter) is the Unicode character set. Unicode overcomes two of ASCII's greatest limitations: the limited character space (i.e., a maximum of 128/256 characters in an 8-bit byte) and the lack of international (beyond the USA) characters.

Unicode uses a 16-bit word to represent a single character. Therefore, Unicode supports up to 65,536 different character codes. This is obviously a huge advance over the 256 possible codes we can represent with an 8-bit byte. Unicode is upward compatible from ASCII. Specifically, if the H.O. 9 bits of a Unicode character contain zero, then the L.O. 7 bits represent the same character as the ASCII character with the same character code. If the H.O. 9 bits contain some non-zero value, then the character represents some other value. If you're wondering why so many different character codes are necessary, simply note that certain Asian character sets contain 4096 characters (at least, their Unicode subset).

This text will stick to the ASCII character set except for a few brief mentions of Unicode here and there. Eventually, this text may have to eliminate the discussion of ASCII in favor of Unicode because many new operating systems are using Unicode internally (and convert to ASCII as necessary). Unfortunately, many string algorithms are not as conveniently written for Unicode as for ASCII (especially character set functions) so we'll stick with ASCII in this text as long as possible.